Owing to the increasing scarcity of non-renewable energy reserves such as coal, petroleum and uranium, it is essential that increased use be made of alternative non-depletable energy sources, such as photovoltaic energy. Single crystal photovoltaic devices, especially crystallline silicon photovoltaic devices, have been utilized for some time as sources of electrical power because they are inherently non-polluting, silent and consume no expendable natural resources in their operation. However, the utility of such crystalline devices has been limited by problems associated with the manufacture thereof. More particularly, single crystalline materials are: (1) difficult to produce in sizes substantially larger than several inches in diameter, (2) thicker and heavier than their thin film counterparts; (3) fragile and therefore susceptible to breakage; and (4) expensive and time consuming to fabricate.
Recently, considerable effort has been expended to develop systems and processes for preparing thin film amorphous semiconductor alloy materials which encompass relatively large areas and which can be deposited so as to form p-type and n-type semiconductor alloy layers for the production therefrom of thin film photovoltaic devices which are substantially equivalent or superior to their crystalline counterparts in operation and efficiency. It should be noted at this point that the term "amorphous" as used herein, is defined to include alloys or materials exhibiting long range disorder, although said alloys or materials may exhibit short or intermediate range order or even contain crystalline inclusions. Also, as used herein, the term "microcrystalline" is defined as a unique class of said amorphous materials characterized by a volume fraction of crystalline inclusions, said volume fraction of inclusions being greater than a threshold value at which the onset of substantial changes in certain key parameters such as electrical conductivity, band gap and absorption constant occur. It is to be noted that pursuant to the foregoing definitions, the microcrystalline, p-doped, wide band gap, semiconductor alloy material, referred to herein, falls within the generic term "amorphous".
As mentioned hereinabove, amorphous thin film semiconductor alloys have gained acceptance for the fabrication of photovoltaic cells therefrom. This is because the amorphous thin film semiconductor alloys (1) can now be manufactured by relatively low cost continuous processes, (2) possess a wide range of controllable electrical, optical, and structural properties and (3) can be deposited to cover relatively large areas. Among the semiconductor alloy materials exhibiting the greatest present commercial significance are amorphous silicon, amorphous germanium and amorphous silicon-germanium based alloys. Such alloys have been the subject of a continuing development effort on the part of the assignee of the present invention. More specifically, the assignee of the present invention is recognized as the world leader in photovoltaic technology. Photovoltaic devices produced by said assignee have set world records for photoconversion efficiency and long term stability under operating conditions (the efficiency and stability considerations will be discussed in greater detail hereinbelow). Additionally, said assignee has developed commercial processes for the continuous roll-to-roll manufacture of large area photovoltaic devices.
In this roll-to-roll processing, a web of substrate material may be continuously advanced through a succession of operatively interconnected, environmentally protected deposition chambers, wherein each chamber is dedicated to the deposition of a specific layer of semiconductor alloy material onto the web or onto a previously deposited layer. In making a photovoltaic device, for instance, of n-i-p type configuration, the first chamber is dedicated for the deposition of a layer of an n-type semiconductor alloy material, the second chamber is dedicated for the deposition of a layer of substantially intrinsic amorphous semiconductor alloy material, and the third chamber is dedicated for the deposition of a layer of a p-type semiconductor alloy material. The layers of semiconductor alloy material thus deposited in the vacuum envelope of the deposition apparatus may be utilized to form photoresponsive devices, such as, but not limited to, photovoltaic devices which include one or more cascaded n-i-p type cells. By making multiple passes through the succession of deposition chambers, or by providing one or more additional triads of deposition chambers, multiple stacked cells of various configurations may be obtained. Note, that as used herein, the term "n-i-p type" will refer to any sequence of n and p or n, i and p layers of semiconductor alloy material operatively disposed and successively deposited to form a photoactive region wherein charge carriers are generated by the absorption of photons from incident radiation.
The concept of utilizing multiple stacked cells, to enhance photovoltaic device efficiency has been known since at least as early as 1955. Essentially, the concept employs different band gap devices to more efficiently collect various portions of the solar spectrum and thereby increase open circuit voltage (Voc). The tandem cell device (by definition) incorporates two or more stacked cells with the light directed serially through each cell. In the first cell, a large band gap semiconductor alloy material absorbs only the short wavelength light, while in subsequent cells, smaller band gap semiconductor alloy materials are employed to absorb the longer wavelengths of light which pass through the first cell. By substantially matching the photogenerated currents from each cell of the tandem arrangement, the overall open circuit voltage becomes the sum of the open circuit voltage of each cell, while the short circuit current thereof remains substantially constant. Such tandem cell structures can be relatively economically fabricated in large areas by employing thin film amorphous, semiconductor alloy materials (with or without crystalline inclusions). It should be noted that when crystalline semiconductor materials are employed for the fabrication of stacked cell structures, it is virtually impossible to match the lattice constants of the different crystalline materials thereof. Therefore, it is not possible to fabricate such crystalline tandem cell structures in a commercially feasible manner. In contrast thereto, and as the assignee of the instant invention has shown, such tandem cell structures are not only possible, but can be economically fabricated over large areas by employing the amorphous semiconductor alloy materials and the deposition techniques discussed and briefly described herein.
More particularly, the assignee of the instant invention is presently able to manufacture stacked, large area photovoltaic devices on a commercial basis by utilizing the previously referenced, continuous deposition, roll-to-roll processor. That processor is characterized as a 1.5 megawatt capacity machine insofar as its annual output of photovoltaic devices is capable of producing 1.5 megawatts of electrical power. Said 1.5 megawatt processor, as presently configured, is adapted to produce tandem photovoltaic cells which comprise two stacked n-i-p type photovoltaic devices disposed optically and electrically in series upon a stainless steel substrate. The processor currently includes six operatively interconnected, dedicated deposition chambers, each deposition chamber adapted to sequentially deposit one of the layers of semiconductor alloy material from which the tandem device is fabricated.
Despite the fact that non-polluting, non-depletable photovoltaic energy is so attractive and despite the fact that Applicants' assignee has been able to develop production apparatus in which thin film photovoltaic devices can be manufactured in a continuous roll-to-roll process, the cost in dollars per peak watt of power generated by thin film photovoltaic devices remains too high to be cost competitive with conventional energy sources. One reason for this relatively high cost is that while the aforementioned continuous roll-to-roll manufacture of thin film photovoltaic devices has reduced the deposition expense as compared to the expense of batch depositing those devices, in order to series connect discrete devices, it has heretofore been necessary to sever said devices from the web and then electrically and mechanically reconnect the severed devices in said series relationship. The processing steps involved in the severing of discrete devices, electrically interconnecting in series those severed devices, and mechanically reconnecting the severed, electrically interconnected devices could not be accomplished in roll-to-roll fashion, was labor intensive, time consuming and hence, expensive.
It is therefore the principle object of the subject invention to provide a continuous, roll-to-roll process for electrically interconnecting in series a plurality of discrete small area photovoltaic cells, which process is simple and cost effective.
This and other objects and advantages of the subject invention will become apparent from a perusal of the drawings, the detailed description of those drawings, and the claims which follow.